Spying Inside Steel

Corrosion wreaks havoc on steel, which is used in everything from kitchen sinks to nuclear reactors. Now researchers led by material scientist Andrew King at the University of Manchester, in England, have employed diffraction contrast tomography (DCT) to take a microscopic peek at corrosion-based crack formation in steel (Science 2008, 321, 349). With DCT, the researchers reconstructed the orientation of the many randomly oriented crystal grains that form steel's microstructure, which then allowed them to investigate the corrosion processes occurring at the grain interfaces. For example, heat from welding can lead to formation of chromium carbide deposits. Diffusion of chromium to interfacial precipitates weakens the remaining steel by making it more sensitive to oxidation, which in turn makes the material more liable to crack. But these precipitates form only at the interfaces between neighboring grains that are oriented in a way that provides space for nucleation of precipitates, King says. Knowing the orientation of steel's component crystals permits researchers to identify the grain boundaries most vulnerable to cracking and could provide guidance for developing steelmaking processes that do not produce such structures.